I have a brand new server containing two identical 3 TB SATA disks. These are in a hardware RAID1 configuration using the onboard Intel RAID controller. The BIOS sees the two disks as 3 TB and the resulting RAID volume is 3TB.
On this server, I am attempting to install Ubuntu Server 12.04 (64-bit). Setup correctly detects the RAID volume, but insists the volume is 801.5 GB. Attempting to change the partition type to GPT does nothing.
If I continue all the way through setup anyway, the system is created with a roughly 800 GB root volume, swap, etc. Running fdisk following this shows the remaining free space on the volume and lets me attempt to partition it. I have not yet tried this, and would really like the entire 3 TB to be part of the root volume. I would also like to avoid attempting to resize the partition after the fact, if at all possible.